io_uring/zcrx: move count check into zcrx_get_free_niov

io_uring/zcrx: move count check into zcrx_get_free_niov

Instead of relying on the caller of __io_zcrx_get_free_niov() to check
that there are free niovs available (i.e. free_count > 0), move the
check into the function and return NULL if can't allocate. It
consolidates the free count checks, and it'll be easier to extend the
niov free list allocator in the future.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/6df04a6b3a6170f86d4345da9864f238311163f9.1774261953.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>